1. Field of the Invention
The invention relates to a nickel-based alloy. Preferably, the alloy is particularly suitable for the manufacture of a welding electrode containing a wire made of a nickel-based alloy which may be used for the welding of products or workpieces, especially sheets and plates, made of metal, and especially of superaustenitic stainless steel or superduplex stainless steel. This alloy may also be used for producing, by welding, corrosion-resistant coatings on products made of low-alloy steel. A welding electrode and a process of welding are also described.
2. Discussion of the Background
Many items of equipment having to withstand corrosion are manufactured by the welding assembly of products such as sheets, plates or tubes made of superaustenitic or superduplex stainless steel. Superaustenitic and superduplex stainless steels are stainless steels containing (in % by weight) especially 18% to 30% chromium, up to 7% molybdenum, up to 0.5% nitrogen as well as nickel, the content of which is adjusted in order to obtain either an entirely austenitic structure or a mixed structure which is partially austenitic and partially ferritic. These steels are characterized by a number PREN=Cr %+3.3Mo %+16N % of greater than 35. This number is an indicator of the resistance to localized corrosion, which resistance is better the higher the number.
In order for equipment manufactured in this way to behave satisfactorily in service, the welds must exhibit both corrosion resistance and mechanical properties such as satisfactory hardness and toughness which are compatible with the properties of welded sheets or plates. In addition, the welds must be able to be produced without them having any defects.
When the conditions of use require that the welds exhibit corrosion resistance as good as the base metal, the products are welded with a filler metal made of a nickel-based alloy of the 625 type containing approximately 22% chromium, 9% molybdenum and 3.5% niobium, the balance being impurities resulting from smelting. In particular, this alloy does not contain nitrogen. This technique has the drawback of causing problems of heat cracking and of composition heterogeneity of the welds.
In order to remedy these drawbacks, it has been proposed to use a niobium-free nickel-based alloy containing from 18% to 25% chromium, from 6% to 12% molybdenum completely or partially replaced with tungsten (6%xe2x89xa6Mo+Wxe2x89xa612%), from 0.1% to 3% copper and from 0.1 to 0.3% nitrogen. This alloy has the advantage of leading to welds which exhibit good hot-cracking resistance, good corrosion resistance and good toughness. However, because of the presence of copper, this alloy is difficult to hot roll and, because of a high nitrogen content, which leads to the formation of nitrides of the (0.5Cr, 0.32Ni, 0.18Mo)5N type, it is difficult to wiredraw. Moreover, at least in the case of superaustenitic steels, the corrosion resistance of the welds is slightly lower than with 625 alloy containing niobium.
In order to remedy the drawbacks resulting from the presence of copper and from a high nitrogen content, it has been proposed to use a nickel-based alloy of the type containing 22% chromium and 9% molybdenum, containing neither niobium nor copper and with a nitrogen content of less than 0.1%. This alloy hot rolls and wiredraws well, and leads to welds which exhibit good hot-cracking resistance. When welding superduplex alloys, this alloy improves the toughness of the welds by limiting segregation and the formation of intermetallic phases. However, in the case of superaustenitic stainless steels, the resistance to localized corrosion of the welds obtained is not as good as would be desired. In addition, in the case of superduplex steels, the hardness of the welds is slightly insufficient.
One object of the present invention is to remedy these drawbacks by providing a nickel-based alloy which can be used for the manufacture of wire for the welding of products made of superduplex or superaustenitic stainless steel, is easy to hot roll and to wiredraw and leads to welds having good hot-cracking resistance, good toughness, satisfactory hardness and good corrosion resistance.
Another object relates to a method of welding sheets or plates of superaustenitic stainless steel or superduplex stainless steel and to a welding method for coating products made of low-alloy steel by addition of metal by melting a welding electrode as defined above.
The invention also relates to the nickel-based alloy of which the welding electrode is composed and whose composition is defined herein. Other objects are apparent from the description above and from that which follows.
The invention nickel-based alloy and welding electrode wire or strip of this nickel-based alloy, has a chemical composition which comprises, consists essentially of, or consists of, by weight:
20%xe2x89xa6Crxe2x89xa624%
8%xe2x89xa6Moxe2x89xa612%
0.1%xe2x89xa6Wxe2x89xa65%
Mo+W greater than 12%
Fe less than 5%
Cu less than 0.1%
N less than 0.1%
C less than 0.02%
Si less than 0.1%
Mn less than 0.4%
0.03%xe2x89xa6Alxe2x89xa60.4%
0.004%xe2x89xa6Mgxe2x89xa60.04%
P less than 0.02%
S less than 0.02%,
nickel and impurities resulting from smelting.
Preferably, the chemical composition is such that:
21%xe2x89xa6Crxe2x89xa623%
9%xe2x89xa6Moxe2x89xa611%
2%xe2x89xa6Wxe2x89xa64%
Fe less than 2%
It should be noted that this alloy can also be used for welding metal in general, including steel products of the 9% nickel type (containing mainly iron and 9% nickel) intended for cryogenic applications. The alloy can also be used especially in the form of sheets, plates, bars, tubes and forgings or form the non-corrodible layer of a clad sheet or plate.
When the welding electrode consists of a wire, this may be bare and the electrode may be used for welding according to the MIG process or to the TIG process. The wire may also form the core of a non-synthetic covered electrode. A nonsynthetic covered electrode, known per se, consists of a metal core covered with materials intended especially to form a slag which has no influence on the content of the main elements of the deposited metal.
Thus, the electrode according to the invention is especially either a bare wire made of an alloy according to the invention or a covered electrode, the core of which is a wire made of an alloy according to the invention, or else a strip made of an alloy according to the invention.